Not applicable
1. Field of the Invention
The invention disclosed broadly relates to data storage devices. More particularly, the present invention relates to techniques for correcting the adverse effects of vibrations on servo systems in data storage devices.
2. Description of the Related Art
Hard disk drives are commonly used for mass storage purposes in computers. In FIG. 1 there is shown a block diagram of the major electro-mechanical components of a disk drive 102. The disk drive 102 has a read/write transducer 104, voice coil actuator 106, recording medium or disk 108, and read/write control electronics 110. There is a constant demand for increase storage capacity and reduced disk access times as new software and new applications become available. The present generation of hard disk drives 102 are designed to operate in a portable, a desk-top and a server environment. In order to meet the demand for lower disk access times, the spindle rotation speed for disk 108 has been on the increase. However this increase in spindle rotation speed has present substantial design challenges to overcome.
These design challenges can be categorized based on their source and the directional effects on the read/write transducer 104. The first problem is vibration which is caused from mass imbalances, physical tolerance limitations and electro-mechanical sources. The vibration manifests itself by moving the read/write transducer 104 back and forth in a direction parallel to the surface of the disk 108. The typical sources of vibration include the disk drive itself, other disk drives and other electro-mechanical drives sharing a chassis such as CD ROM drives, diskette drives, and tape drive devices. Vibration produces poor settle out and/or reduces track following characteristics, especially for track densities of 10,000 tracks per inch (TPI) and beyond. Servo systems reduce the track-follow error by about 20 to 30 dB using basic servo loop error rejection properties. Another technique for reducing the effects of vibration is disclosed in application Ser. No. 08/119,181 by S. M. Sri-Jayantha et al. entitled xe2x80x9cAdaptive Vibration Control For Servo Systems In Data Storage Devicesxe2x80x9d filed on Jul., 20, 1998 and is commonly assigned herewith to IBM.
The second problem is flutter of the disk platter 108. The flutter is the movement of the disk 108 that causes track mis-registration error (TMR). Disk-flutter, is primarily caused by pressure fluctuations associated with internal turbulent airflow. The airflow is the result of aerodynamic effects between the fast rotating surface of the disk 108 causing air to disturb the close flying read/write transducer 104. As the rotating speed of a disk is increased from 5,400 rpm to 10,000 rpm, the aerodynamically induced disk-flutter becomes a major contributor to track mis-registration error. The track density of present generation drive is about 15,000 TPI. About 30% of the TMR budget is consumed by disk-flutter effects at disk speeds 7,200 rpm in a disk drive with conventional servo-mechanics configuration. As track density is increased, the disk-flutter-based TMR is expected to contribute well above 30% of the allowed TMR budget unless a cost effective solution is found.
The impact of disk-flutter on TMR can be minimized by aerodynamic redesign of the base-plate, improved stiffness and damping of disk platter substrate, or by novel servo method. Present 3.5xe2x80x3 disk drives have reached the TMR limit posed by the disk-flutter mechanics. Disk-flutter has been observed in the high track density 3.5xe2x80x3 products. The disk-flutter problem can be tackled from three technical viewpoints: mechanical, aerodynamic, and servo.
A higher bandwidth servo system can effectively compensate for the disk-flutter, but no cost effective methodology has been proposed by the storage industry to increase the servo bandwidth without increasing the component count. The strength of the airflow disturbance can be reduced by means of shrouding. However, complex shrouding or machining operations makes the mechanical approach not economical. Moreover, the shrouding must be custom designed for each type and model of hard disk drive 102. One approach can be found in the prior art has been to reduce airflow disturbance in the disk enclosure is in U.S. Pat. No. 4,583,213 by Allen T. Bracken et al. for xe2x80x9cAir shroud for data storage disksxe2x80x9d, issued Apr. 15, 1986. Accordingly, a need exists to reduce fluttering in disk drives without the need of redesigning custom shrouding or custom base plates.
Another approach to reduce disk-flutter is by use of an alternate disk platter substrate with increased stiffness and damping properties. However, new substrates call for investments in research and development. Therefore, a need exists for a method and apparatus to reduce the effects of disk-fluttering TMR without the need of new disk substrates.
Still another approach to reduce disk-flutter problems is to use smaller diameter disk drives. Due to the lack of cost effective solutions for larger diameter hard disks, the storage industry has moved towards 3.0xe2x80x3 and 2.5xe2x80x3 diameter disks to minimize the severity of disk-flutter problems. The effect of mechanical movement of the data tracks due to disk-flutter can be effectively track-followed by increasing the servo bandwidth of a head positioning system. Using micro actuators the bandwidth of a conventional head positioning system can be increased. Examples of micro actuators are found in U.S. Pat. No. 5,657,188 by Ryan Jurgenson et al. for xe2x80x9cHead suspension with tracking microactuatorxe2x80x9d issued Aug. 12, 1997 and U.S. Pat. No. 5,189,578 by Kenji Mori for xe2x80x9cDisk system with sub-actuators for fine head displacementxe2x80x9d issued Feb. 23, 1993. But using micro actuators can add cost to the servo assembly. Accordingly, a need exists to reduce the effect of disk-fluttering TMR without the need to reduce the diameter of the disk and the need to use new forms of disk actuators.
Briefly, in accordance with the present invention, a rotating media storage system comprising: a servo feedback loop for providing a position error signal to position read/write transducer over a rotating media storage; a detector for detecting at least one of the disk-flutter modes induced on the transducer and a narrow band filter is set to servo compensate the fundamental mode of at least one flutter mode.
In another embodiment, the disk-flutter mode is minimized using a lead-lag filter which reduces the amplification of the disk-flutter in the flutter enhancement zone.
In accordance with another embodiment of the present invention, a method that corresponds to the above rotating media storage system is disclosed.